This invention relates to a process of forming rounded vitreous beads in which particles of bead-forming material entrained in a gas stream having comburent and combustible components, are projected from a burner head and the gas is burnt. The invention includes apparatus for use in performing such a process.
Various proposals have been made for the manufacture of vitreous beads, such as that set forth in French Patent specification No. 1,161,396 (Centre National de la Recherche Scientifique). As described in that specification, air in which bead forming particles are entrained is fed to a combustion chamber via a burner head along a conduit which is surrounded by two other concentric conduits each carrying a combustible gas/air mixture. In order to promote mixing of the particle entraining air stream and the gas/air streams, that conduit which immediately surrounds the inner one is so shaped as to cause the gas/air mixture flowing along it to swirl as it emerges at the burner head. It has been found however that the mixing in the combustion chamber is not efficient and that the bead forming particles still tend to be carried along in a central air stream which is surrounded by a combustible mixture with the result that when the gas is ignited, the outer particles tend to shield the inner particles from the heat produced and the yield of well formed beads is not as high as would be desired.